Digital optical discs are now being seriously considered as alternatives to magnetic tapes and disc memories. It has been found that optical discs offer substantially greater data storage capacity than commercially available magnetic tape or disc memories of similar size.
For optical disc recording or play back, accurate focusing of the laser beam on the storage medium's surface is essential. Once focus is obtained, it is required that the focus point be maintained in spite of vertical run-out variations of the disc surface. This is achieved by means of a focus-following type servo mechanism controlling the vertical positioning of the objective lens. However, the servo assumes that the spatial position of the true focus point is coincident with the zero of the differential error curve of the focus sensors. This condition for coincidence can be upset by changes in the mechanical alignment of the optics and/or the d.c. stability of the electronic circuitry. The result is a focus offset which, if not corrected, produces a loss of spot resolution and accuracy thus degrading the performance of the whole system. A means of correcting for this drift problem is required and a solution to the problem is the subject of this invention.